If my assumption is correct then you have to right click onto the cuboid you want to add a thermal contact resistance -> click on "Surface...". On the new opened window "Surface Attribute" enter the value for the "Rsurf-solid" [K*m^2/W].

E.g.: If you have 4µm air@25°C gap between an aluminum cuboid and the TIM you have to add a Rsurf-solid equal to (4*10e-6)[m]/(0.026)[W/m*K]=1.53e-4[K*m^2/W] on the surface of the aluminum that is in contact with the ThermalInterfaceMaterial.